2.1. Magnetic Resonance Imaging
In magnetic resonance imaging (MRI) an image of an organ or tissue is obtained by placing a subject in a strong magnetic field and observing the interactions between the magnetic spins of the protons and radiofrequency electromagnetic radiation. (For a review of MR imaging technique see Balter, S. RadioGraphics 1987, 7(2) 371-383; Fullerton, G. D. RadioGraphics 1987, 7(3), 579-596). Two parameters termed proton relaxation times are of primary importance in the generation of the image. They are called T.sub.1 (also called the spin-lattice or longitudinal relaxation time) and T.sub.2 (the spin-spin or transverse relaxation time). T.sub.1 and T.sub.2 depend on the chemical and physical environment of protons in various organs or tissues.
The utility of MR imaging techniques in the characterization and differentiation of pathologic from healthy tissues is most easily demonstrated in cases where divergent relaxation times occur within a region of interest. For example in cerebral tissue the protons of the cerebral spinal fluid have far different relaxation times from neural tissue and the resulting MR images are of high contrast.
In other instances the image produced may lack definition and clarity due to a similarity of the signal from different tissues or different compartments within a tissue. In some cases, the magnitude of these differences is small, limiting the diagnostic effectiveness of MR imaging. Thus, there exists a real need for methods which increase or magnify these differences. One approach to improving image quality is through the use of contrast agents.